Generally, a blow molding machine used for producing PET plastic bottles is comprised of a feeding unit, a transportation unit, a heating unit, a blow molding unit and a finished product ejection unit. Operationally, bottle embryos are fed into the transportation unit by the feeding unit for carrying the same to the heating unit to be heated to a specific temperature, and then the heated bottle embryos are inserted into the molds of the blow molding unit for blow molding, and after blowing molding, the bottle embryos are transformed into finished products which are to be served by the finished product ejection unit.
For enhancing production efficiency, the blow molding unit usually are configured with more than one mold to be used for blow molding more than one bottle embryos into finished products at the same time. However, as there are a batch of bottle embryos to be inserted into the molds of the blow molding unit all at the same time, it is important to have the pitches between any two neighboring bottle embryos to be adjusted matching to those between their corresponding molds, so that the bottle embryos of the same batch can be properly inserted into their corresponding molds after being fed into the blow molding unit.
For matching the pitches between any two neighboring bottle embryos with the pitches between their corresponding molds, some blow molding machine adopts a kind of transportation unit that is capable of transporting a plurality bottle embryos one after another while spacing any two neighboring bottle embryos by a distance matching to the fixed pitch measured between their corresponding molds, as the one shown in TW Pat. No. 365573.
However, as there are fixed pitches between molds, the bottle embryos in the molds that are being transported by the transportation unit are being fed into the heating unit separately one after another in a manner that as soon as a bottle embryos enters the heating unit, the transportation unit will travel a distance equal to the pitch between the molds before the following bottle embryos can be fed into the heating unit so that the heating efficiency of the heating unit is considered to be adversely affected. One the other hand, if the bottle embryos are being fed into the heating unit one immediately after another without any delay, the heating unit will have better heating efficiency.
Consequently, there are some conventional blow molding machine adopts the design for feeding the bottle embryos into its heating unit one immediately after another, as the one disclosed in TW Pat. No. 453945.
Although the heating efficiency of the blow molding machine with ability to fed its bottle embryos into its heating unit one immediately after another is comparatively better, it will require to have an additional device to enlarger the interval between any two neighboring heated bottle embryos for matching with the pitch between their corresponding mold for preparing the heated bottle embryos to be sent to the blow molding unit. It is noted that such additional pitch enlarging unit is also disclosed in TW Pat. No. 453945.
Please refer to FIG. 1 to FIG. 3, which is a top view and a side view of a pitch enlarging unit for conventional blow molding machines, and a schematic view showing the operation of the pitch enlarging unit.
As shown in the figures, the pitch enlarging unit 8 is mounted on a guide rail 711 of a transportation unit 71 for enabling the same to travel between a heating unit 72 and a blow molding unit 73. In FIG. 2, the pitch enlarging unit 8 is disposed at the tip of a supporting arm 91 mounted on a base 9, which is configured with a supporting plate 81, being arranged for enabling the same to be driven to move reciprocatively by a direct-acting cylinder 82 in a direction perpendicular to the guide rail 711.
There is a linear rail 83 mounted on the supporting plate 81 for the four carrying elements 841, 842, 843, 844 to mount thereon; and there is a retractable mechanism 85 being disposed on the pitch enlarging unit 8 in a direction parallel with the guide rail 711. It is noted that the retractable mechanism 85 has a direct-acting cylinder 86 which is mounted on the supporting plate 81 and is used as a power source for driving the retractable mechanism 85 to extend or retract. Moreover, the four carrying elements 841, 842, 843, 844 are pivotally connected to the four joints 851, 852, 853, 854 in respective while the carrying element 841 is further being secured fixedly on the supporting plate 81, and the piston rod 861 of the direct-acting cylinder 86 is connected to the carrying element 841. As shown in FIG. 2, there is a groove 845˜848 being formed at the tip of each of the four carrying elements 841, 842, 843, 844, which is used for receiving the carrier 74 carrying a bottle embryo P as the carrier 74 is being transported by the transportation unit 71.
Operationally, as soon as there are carriers 74 with bottle embryos P to be sent to the pitch enlarging unit 8 by the transportation unit 71, the direct-acting cylinder 82 is activated for pushing the supporting plate 81 toward the guide rail 711 so as to prepare the grooves 845˜848 of the carrying elements 841, 842, 843, 844 for receiving carriers 74 in respective, as shown in FIG. 2.
Thereafter, another direct-acting cylinder 86 will drive the retractable mechanism 85 to extend for bringing along the carrying elements 841˜843 to slide along the linear rail 83 and thus causing the pitches between any two neighboring carriers 74 to enlarge, as shown in FIG. 3.
Then, the direct-acting cylinder 82 is acted to pull the supporting plate 81 backward for detaching the carrying elements 841˜844 from their corresponding carriers 74 for enabling the carriers 74 to be sent to the blow molding unit 73 by transportation unit 71, i.e. as soon as the pitches between any two neighboring carriers 74 are enlarged, the carriers 74 will be released from their corresponding carrying elements 841˜844 for enabling the same to be sent to the blow molding unit 73 by transportation unit 71. Finally, the direct-acting cylinder 86 will drive the retractable mechanism 85 to retract so as to bring the carrying elements 841˜844 back to their initial positions, i.e. the four carrying elements 841˜844 are disposed one immediately after another, and thus complete a cycle of pitch enlarging operation.
However, for completing a cycle of pitch enlarging operation, the aforesaid pitch enlarging unit 8 must be designed with a structure capable of being driven by the two direct-acting cylinders 82, 86 to perform a moving operation sequentially and reciprocatively following a rectangle-shaped path as the path is defined by the direction parallel to the guide rail and the direction perpendicular to the guide rail, so that the overall structure of such pitch enlarging unit 8 can be very complex.
In addition, although the pneumatic type direct-acting cylinder is quick in response, it is disadvantageous in that it lacks the ability to position accurately. To make the matter worse, the aforesaid pitch enlarging unit 8 has two direct-acting cylinders 82, 86 which are acting in different directions and can exert influence on each other, so that pitch enlarging unit 8 may suffer severe positioning inaccuracy problem. Moreover, since the grooves 845˜848 of the carrying elements 841, 842, 843, 844 are formed as open structures, they can be hold on to their corresponding carriers 74 firmly and exactly. Inevitably, the positioning inaccuracy and the open-structured grooves 845˜848 are going to cause problems to the carrying elements 841˜844 when they try to receive carriers 74 from the transportation unit 71 or to release carriers 74 to the transportation unit 71.
Moreover, as the retractable mechanism is composed of a plurality linkage rods that are pivotally connected with each other, it has many joints formed in the mechanism. Taking the aforesaid retractable mechanism 85 with four carrying elements for example, there are 10 joints in the structure of the mechanism. It is noted that too many joints in the structure may cause inaccuracy to any extending or retracting operation of the mechanism.
To sum up, despite that the pitch enlarging unit disclosed in TW Pat. No. 453945 is quick in response, it is disadvantageous in that it is too complex in structure and it lacks the ability to position accurately.
For solving the aforesaid problems, there is another improved pitch adjusting device being provided in TW Pat. Appl. No. 97203621. The improved pitch adjusting device is comprised of: a screw rod, a driving unit for driving the screw rod, a plurality of clipping units and an ejection mechanism. The screw rod is extending along the transmission direction of the transportation unit and reaches a blow molding unit, in which the small pitch section of the screw rod, being the portion of the screw rod next to the transportation unit, is formed with a plurality of guide slots in a manner that the pitches measured between any two neighboring guide slots at such small pitch section are equal to those between the corresponding two neighboring bottle embryos as they are being transported by the transportation unit; and the large pitch section of the screw rod, being the portion of the screw rod next to the blow molding unit, is formed with a plurality of guide slots in a manner that the pitches measured between any two neighboring guide slots at such large pitch section are equal to those between the corresponding two neighboring molds of the blow molding unit; and the large pitch section is connected to the small pitch section by a variable pitch section. The plural clipping units are disposed at positions corresponding to disposition of guide slots on the screw rod. As each clipping unit is used for holding a bottle embryo and is driven to move by the screw rod, the clipping units, each holding a bottle embryo, are gradually being transferred from the large pitch section to the small pitch section of the screw rod so that the pitches between bottle embryos are varied with pitch variation between the guide slots, and thus the pitches between embryos can be adjusted during the transportation between the transportation unit and the blow molding unit.
However, there are still disadvantages existed in the aforesaid pitch adjusting device being provided in TW Pat. Appl. No. 97203621, which are described in the following:                (1) As both the small pitch section and the large pitch section of the screw rod must be designed with sufficient lengths for the plural clipping units to disposed about, consequently, the overall length of the screw rod is enlarged that is going to cause additional problems, such as the stroke of the whole blow molding operation is prolonged and the time required for pitch adjustment is increased.        (2) The aforesaid guide slot with variable pitch design can only be realized on a screw rod with large enough diameter. However, the thicker the screw rod is, the more costly it is to be manufactured, not to mention that the response of such thicker screw rod will be adversely affected as it is going to be more heavier.        (3) As the clipping units holding the bottle embryos are being transferred between the small pitch section, the variable pitch section and the large pitch section, the connections between such transferring could be problematic when it comes to noise.        
Therefore, it is in need of a pitch adjustment device for blow molding machine for overcoming the aforesaid problems.